This invention relates to an instrument hanger sub and, more particularly, to a pipe sub for receiving an acoustic instrument and positioning the instrument in a string of pipe in a manner affecting the efficient transmission of longitudinal sound waves in the pipe string for use in a borehole. The present invention results from developmental work on a wellbore telemetry system and, in particular, a drill pipe acoustic telemetry system. The need for means to transmit downhole data to the surface during a drilling operation is well-known in the petroleum and mining industries. In recent years, with the advent of deeper drilling operations and technical innovations which permit the detection of downhole parameters useful at the surface during the drilling operation, the need for such a telemetry system has increased.
During the course of drilling oil and gas wells, for example, there is a need for transmitting data from the bottom of the borehole to the surface without the use of electrical transmission cables. One of the most important needs for such communication is in directional drilling operations where it is helpful for the driller to know at all times the orientation of the drill in order to direct the hole in a desired direction and with a desired inclination from the vertical. Such a directional drilling system, utilizing an electrical transmission cable to transmit data to the surface, is set forth in U.S. Pat. No. 3,935,642. In such systems, rotation of the bit is typically actuated by means of a mud powered motor positioned just above the bit in the drill string. Immediately above the mud motor is a bent sub which is usually a small section of pipe having a bend of one to three degrees in its longitudinal axis. Above the bent sub is a string of drill pipe reaching to the surface which serves to support the drilling tools and to conduct a high pressure stream of drilling fluid to the downhole mud motor to power the motor and wash bit cuttings from the borehole. Downhole instruments, if used for detecting borehole parameters, are typically positioned in the drill string near the bottom of the string, and, it is desirable to transmit this information to the surface so that changes in the drilling operation may be effected in response to the measured downhole parameters. Instrumentation, for example, for sensing and transmitting the information regarding the orientation of a drill bit, such as shown in U.S. Pat. No. 3,935,643, is available commercially. However, such present commercial equipment utilizes an insulated electrical cable run inside the drill pipe as a means of supplying power to the downhole equipment and for transmitting data to the surface. The need for thousands of feet of armored electrical cable requires a large capital investment for the cable and hoisting equipment. Equally important, however, is the loss of drilling time resulting from the fact that it is necessary to withdraw the entire cable from the hole each time a joint of pipe is added to the drill string at the surface and then to replace the instrument and cable into its operating position in the pipe before drilling is resumed. The situation above described illustrates the long felt need for a wireless data transmission system. This need has led to extensive research in the area of downhole telemetry systems.
One system presently being developed is popularly referred to as a mud pulse system wherein the circulating stream of drilling fluid is modulated by pressure pulses to transmit data. The mud pulse system, however, involves complex hardware and provides a slow data rate compared to proposed acoustic drill pipe systems. Such proposed acoustic drill pipe systems are set forth in co-pending patent applications Ser. No. 390,833, entitled "Telemetry System for Boreholes" and serial number 775,620, entitled "Telemetry System". U.S. Pat. No. 3,930,220 also shows an acoustic drill pipe telemetry system. Another co-pending patent application, Ser. No. 774,432, entitled "Acoustic Transducer" shows an acoustic transducer which serves as a receiver and sound source and is adaptable to the acoustic instrument section which is disclosed schematically in the present application. The systems disclosed in the above applications and patent, involving the transmission of acoustic signals, deal primarily with the transmission of a signal through the steel body of a drill pipe string. There are problems, however, involving the transmission of acoustic signals in a pipe string. Many of these problems are set forth in the above-referenced applications and patent. For example, there are serious losses of acoustical energy due to attenuation in the drill string. Also, the elimination of electrical communication to the surface means that downhole instrumentation must be battery powered or powered by a mud generator, both of which must be of limited size in order to fit into the drill string. Therefore, it is desirable that an acoustic telemetry system utilize a sound source, a sound receiver, and other downhole equipment systems such as an instrument housing for encasing the receivers and transmitters, and, an instrument hanger for positioning the housing in the pipe string in such a manner as to provide a highly efficient coupling of the acoustic energy between the instrumentation and the steel drill pipe. Co-pending patent application, Ser. No. 774,432, entitled "Acoustic Transducer" discloses a magnetostrictive sound transducer which is adapted for use in a telemetry system, with this application setting forth in detail the problems involved in transmitting signals through a string of drill pipe and the necessity for developing as efficient a system as possible to permit the trouble free transmission of signals over a drill pipe.
One of the problems encountered in developing an acoustic telemetry system involves a sub or pipe section for hanging an acoustical instrument in the pipe string in such a way as to permit the flow of drilling fluids about the instrument and at the same time provide an efficient sound transmission path between the acoustic instruments and the pipe sub which are connected in the drill string.
The functions of an instrument sub for use in such a system are
(1) to provide a structural member to connect and transmit tension and torsional forces between adjacent sections of drill pipe; PA1 (2) to provide firm support for the instrument package; PA1 (3) to provide ample passages for the flow of drilling fluid around the instrument assembly; and PA1 (4) to provide an efficient acoustic path for transfer of sound to and from the drilling pipe to the acoustic telemetry instrument.
A conventional instrument sub typically includes a section of pipe having an open bore in which tubular instrument seating members are positioned and connected to the interior walls of the pipe section by means of spiders, or struts, which are radially positioned at right angles between the tubular member and the interior wall of the pipe section. These struts provide for a minimum of interference of fluid flow path through the pipe section and, in addition, because they are usually welded to the tubular member and the interior of the pipe section, provide a firm support for the instrument package. The pipe section is normally threaded at its end to provide an acceptable way of transmitting tension and torsional forces between adjacent sections of the pipe string. Such a conventional instrument sub, however, does not provide an efficient acoustical path for the transfer of sound between the drill pipe and the acoustic instrument. The reason that the efficient acoustical path is not provided is that longitudinal sound waves in a pipe can pass through the arcuate walls of the pipe section between the connecting struts, in a path having a much greater area, and therefore, lower acoustical impedance than the path permitted through the struts; that is, the path from the exterior wall of the pipe section through the struts into the tubular member positioned within the pipe string for holding the acoustic instrument. Additionally, the sound transmitted through the struts is by shear forces in the strut. This produces a bending force in both the tubular seating member and the pipe wall tubes represented by the tubular seating member and the pipe wall. The tubes are much less rigid in this transverse bending direction than along their longitudinal axis. Hence, the sound energy is converted to a lateral vibration mode rather than producing vertical oscillations of the instrument and pipe.
In any event, prior art instrument housing subs are not adequate to provide a sufficient acoustical path to permit the successful operation of a downhole telemetry system. It is therefore the object of the present invention to provide a sub for housing an acoustic instrument and to provide an efficient sound path between the acoustic instrument and a pipe string in a borehole telemetry system.